The separation of many classes of compounds by selective adsorption on molecular sieves or zeolites as well as other adsorbents is well known. For example, as disclosed in U.S. Pat. No. 4,048,205 to Neuzil et al., methyl esters of fatty acids of various degrees of unsaturation may be separated from mixtures of esters of saturated and unsaturated fatty acids with X or Y zeolites exchanged with a selected cation. Further, in U.S. Pat. No. 4,353,838 to Cleary. et al., it is disclosed that monoethanoid fatty acids may be separated from diethanoid fatty acids with cross-linked polystyrenes, e.g. "Amberlite".
In a disclosure directly related to this invention, U.S. Pat. No. 4,277,412, the fractionation of fatty acid triglycerides by an adsorption process with permutite adsorbent and desorbent having specific solubility parameters is disclosed. The process, however, is not practical in many applications, since it requires the removal of certain contaminating adsorbent-deactivating reactants; otherwise, this adsorbent's life is extremely, and decidedly uneconomically, short, i.e., several hours or a day or two, at most.
U.S. Pat. No. 4,284,580 has a similar disclosure except that the adsorbent is surface-aluminated silica gel.
Neither suggests or discloses any reaction mechanism that might be responsible for deactivating the adsorbent. Furthermore, neither suggests or teaches a method for regenerating the adsorbent, either batchwise or continuously.
U.S. Pat. No. 3,696,107 discloses a process for separating para-xylene from a feed stream containing a mixture of C.sub.8 aromatics which employs the basic processing steps described in the first patent, a particular crystalline aluminosilicate adsorbent and a two-stage desorption operation in which a first desorbent stream contacts adsorbent in the desorption zone to effect the desorption of paraxylene from the adsorbent and a second desorbent stream contacts the adsorbent in the desorption zone to effect the pushing of desorbed para-xylenes from the interstitial void spaces between the adsorbent particles. One extract stream is withdrawn from the process.
U.S. Pat. Nos. 3,723,302, 4,006,197 and 4,036,745 each disclose a similar process for separating n-paraffins from isoparaffins using dual desorbents. In U.S. Pat. No. 4,006,197, sidecut streams taken from both extract and raffinate fractionator desorbents removed therein are recycled to the process. In this case, only one extract stream is removed. First and second desorbents are introduced, respectively, in zone II and between zones III and IV in U.S. Pat. Nos. 4,006,197 and 4,036,475. In U.S. Pat. No. 3,723,302, both desorbents enter into the desorption zone, but two extract streams are withdrawn. In U.S. Pat. No. 4,036,745, one or two extract streams may be withdrawn.